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Furthermore in the case of Firedrives weapon mode, it says the upgraded character gets the attack bonus, so will the character keep the attack bonus, just not the +3 since the weapon will be scrapped?

Yes. The character will keep the Attack bonus but not the +3.
Source: Wizards of the Coast (2019-11-22)    【Link to Ruling】
Tags: Private Firedrive, Ground Command • Artillery